There has been a growing tendency to include lipid products containing polyunsaturated fatty acids derived from fermentation, processes in various foodstuffs. Of importance is the recently established need to incorporate polyunsaturated fatty acids in infant formula.
Various processes have been described for the fermentative production of lipid or oil containing polyunsaturated fatty acids. Examples are EP-A-0155420 for the production of .gamma.-linolenic acid- (GLA) containing lipid from Mortierella; EP-A-0223960, EP-A-0276541 and WO-A-92/13086 for the production of arachidonic acid- (ARA) containing oil from Mortierella and/or Pythium; WO-A-91/07499 and WO-A-91/11918 for the production of docosahexaenoic acid-(DHA) containing oil from Crypthecodinium cohnii or Thraustochytrium, and WO-A-91/14427 for the production of eicosapentaenoic acid-(EPA) containing oil from Nitzschia; and U.S. Pat. No. 5,539,133 for production of ARA and EPA from microalgae.
Typically, a microbial species producing a lipid containing the desired polyunsaturated fatty acid(a) is cultured in it suitable medium, the biomass in then harvested and pretreated to enable subsequent extraction of lipid from the microbial biomass with a suitable solvent. The thus-extracted lipid is in a crude form and so is often subjected to several refining steps.
The pretreatment of the wet biomass cake is usually by drying, such as spray drying or lyophilization and/or by mechanical disintegration, such as homogenisation or milling. Drying of the biomass is desirable in order to reduce the amount of solvent and to prevent troublesome emulsions. If all oxidation- and thermo-sensitive lipid, such as a polyunsaturated fatty acid-containing lipid, needs to be isolated, special care needs to be taken to ensure that exposure to unfavourable conditions, which stimulate oxygen-induced degradation, is avoided as much as possible. However, the biomass pretreatment methods used in the art do not avoid such unfavourable conditions.
Yamada et al, Industrial applications of single cell oils, Eds. Kyle and Ratledge, 118-138 (1992) describe an arachidonic acid-containing oil purified from Mortierella alpina with a triglyceride content of 90%. In the recovery process, the harvested biomass is dried and crushed by a ball mill prior to hexane extraction. This method also does not minimise exposure to unfavourable conditions.
Thus, polyunsaturated fatty-acid-containing lipids isolated from microbial biomass according to methods known in the art are exposed to oxidation-stimulating conditions which negatively affect the quality of the oil.